Master/slave transceiver power back-off

ABSTRACT

An apparatuses and methods of setting power back-off of a master transceiver and a slave transceiver is disclosed. One example of a method includes the master transceiver determining a master power back-off, and the slave transceiver determining a slave power back-off based on signals received from the master transceiver, and based on the master power back-off. One example of an apparatus includes a master transceiver and slave transceiver system. The slave transceiver is connected to the master transceiver through a cable. The master transceiver includes means for determining a master power back-off. The slave transceiver includes means for determining a slave power back-off based on signals received from the master transceiver, and based on the master power back-off.

RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 11/438,177 filed May 22, 2006, titled “Master/Slave TransceiverPower Back-Off”.

FIELD OF THE INVENTION

The invention relates generally to communication systems. Moreparticularly, the invention relates to setting master and slavetransceiver power back-off.

BACKGROUND OF THE INVENTION

High-speed networks are continually evolving. The evolution includes acontinuing advancement in the operational speed of the networks. Thenetwork implementation of choice that has emerged is Ethernet networksphysically connected over twisted pair wiring. Ethernet in its BASE-Tforms is one of the most prevalent high speed LANs (local area network)for providing connectivity between personal computers, workstations andservers.

High-speed LAN technologies include 100BASE-T (Fast Ethernet) and1000BASE-T (Gigabit Ethernet). Fast Ethernet technology has provided asmooth evolution from 10 Megabits per second (Mbps) performance of10BASE-T to the 100 Mbps performance of 100BASE-T. Gigabit Ethernetprovides 1 Gigabit per second (Gbps) bandwidth with essentially thesimplicity of Ethernet. There is a desire to increase operatingperformance of Ethernet to even greater data rates, such as specified by10 GBASE-T.

FIG. 1 shows a block diagram of an Ethernet system. This system includesswitches 110, 115, a server 120, a set of blade servers 140, and apersonal computer 145. Each of the Ethernet devices 110, 115, 120, 140includes Ethernet transceivers 130 which enable transmission of Ethernetsignals between the devices 110, 115, 120, 140. The signal transmissionis over Ethernet transmission channels that are provided by cables andconnectors, such as cable 150 and connector 155.

Cables and connectors located proximate to each other can suffer fromcoupling of signals from one cable and connector to another cable andconnector. The coupling is referred to as crosstalk, and is undesirablebecause crosstalk signals can interfere with intended transmissionsignals. Crosstalk signals become more prominent as transmissionfrequencies increase. Therefore, as Ethernet system progresses to 10GBASE-T, crosstalk signals become a greater problem.

One way to reduce the effects of crosstalk is to reduce the signal levelof transmission signals. Lower amplitude transmission signals result inlower amplitude crosstalk, and therefore, cause less interference.However, transmission signal amplitude reduction can cause otherproblems. The transmission can be made unreliable by reducing thetransmission signal amplitude because the SNR (signal to noise ratio) istypically decreased.

One proposed method of reducing transmission signal amplitude tominimize crosstalk is to determine the worst case crosstalk within anetwork. The signal amplitude of all transmitters within the network isreduced until the worst case crosstalk meets a predetermined threshold.The amplitudes of transmission signals of the entire network are reducedto the worst case, to ensure that the crosstalk of the entire networkmeets the crosstalk threshold. This solution is inefficient, however,because the transmission channels (cables) of most of the network canhave much greater transmission signal amplitudes without causingcrosstalk problems. As a result, data transmission within the systemsuffers excessively.

It is desirable to have a system, apparatus and method of adjustingtransmission signal amplitudes (power back-off) of a master transceiverand a slave transceiver within the network. The power back-off isdesirably adjusted to allow the transmission signal amplitude to be aslow as possible while still meeting the desired SINR (signal tointerference and noise ratio) threshold necessary to establish areliable communication link.

SUMMARY OF THE INVENTION

An embodiment includes a method of setting power back-off of a mastertransceiver and a slave transceiver, the master transceiver and theslave transceiver being connected by a cable. The method includes themaster transceiver determining a master power back-off, and the slavetransceiver determining a slave power back-off based on signalstransmitted by the master transceiver, and based on the master powerback-off.

Another embodiment includes a system. The system includes a mastertransceiver, and a slave transceiver, wherein the slave transceiver isconnected to the master transceiver through a cable. The mastertransceiver includes means for determining a master power back-off. Theslave transceiver includes means for determining a slave power back-offbased on signals received from the master transceiver, and based on themaster power back-off.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network of wired devices that suffer from crosstalkbetween cables and connectors between the devices.

FIG. 2 shows one example of a master transceiver and a slave transceiverconnected by a cable (for example, four twisted pairs of copper wire)that can benefit from methods of setting power back-off.

FIG. 3 is a flow chart showing steps of one example of a method ofsetting power back-off of a master transceiver and a slave transceiver.

FIG. 4 shows a plot of power back-off settings including minimum powerback-off, maximum power back-off and ranges of power back-off.

FIG. 5 is a flow chart showing steps of another example of a method ofselecting master and slave transceiver back-offs.

FIG. 6 is a flow chart showing steps of one example of a method ofsetting power back-off of a slave transceiver.

DETAILED DESCRIPTION

The invention includes an apparatus and method for setting powerback-off of a master transceiver and a slave transceiver within anetwork. The power back-off of both transceivers can be adjusted toimprove crosstalk between links of the network.

It is to be appreciated that the present teaching is by way of example,not by limitation. Therefore, although the embodiments described hereinare for convenience of explanation, it is to be appreciated that theprinciples herein may be applied equally to other configurations oftransmitter power back-off methods.

Ethernet systems include multiple high-speed ports that can potentiallyinterfere with each other. These systems include links (cables andconnectors) of varying lengths. The links are typically bundledtogether, may cause long and short links to be proximate when includedwithin a common bundle. As a result, transmitters associated with shortlinks typically transmit signals that interfere (generally referred toas alien crosstalk) with signals being transmitted over long links. Atypical worst cased alien crosstalk scenario occurs when short channelincludes a transmitter (alien disturber) located very close to areceiver (victim) of a very long channel. Transmission signals tend toattenuate proportional to the length of the link that the signals aretraveling over. In the situation where a short link is located near along link, the transmitted signal of the long link can be very weak bythe time it reaches the intended receiver, whereas the alien signal(alien near end crosstalk (ANEXT) and alien far end crosstalk (AFEXT))of the short link can be strong. To minimize the alien signal crosstalk,the transmitter of the short link can reduce its transmission signalpower through power back-off. A link partner of the short link cantypically operate with a lower transmitted signal power because lessattenuation is experienced by the transmission signals of the shortlink. The power back-off, however, reduces the ANEXT and AFEXT of thelong link caused by the short link.

The power back-off setting is typically determined by a transceiverbased on signals received from a link partner. The power back-off isgenerally selected based on signal strength of the received signals,and/or the amount of noise and interference (crosstalk) of the receivedsignals. When transmitting similar power level signals in bothdirections of link partners, the received signal at each link partnertransceiver may be similar, but the power back-off estimates may belimited in quality. For example, for a full-duplex system, the powerback-off can be determined while each transmitter is simultaneouslytransmitting signals, and the received signals may include relativelylarge amounts of local return signals (echo and NEXT). The presence ofthe NEXT and echo signals are likely to reduce the quality of the powerback-off determinations.

The master transceiver and the slave transceiver being subjected todifferent levels of ANEXT and AFEXT interference can also lead toasymmetrical power back-off settings. Since the power back-off settingsare selected based on (among other parameters) measured SINR values,varying the number of active ports (other transceivers) proximate to themaster and slave transceivers generally result in different powerback-off settings for the master transceiver and the slave transceiver.Additionally, once the master and slave transceivers are embedded as acommunication port of a specific application platform (such as a switch,server, or network interface card) the transceivers typically assumeworst case alien crosstalk since the density of adjacent ports can varygreatly in these applications. Generally, in these systems the powerback-off is estimated with a range because the transceivers typicallycannot determine the amount of additional crosstalk that adjacent portsgenerate once the ports are activated. The range can vary depending onthe application in which the transceivers are used.

Asymmetrical Links

The power back-off determinations of a pair of transceivers of abi-directional link can be quite different. That is, a first transceiverof the pair may determine a power back-off that is very different thanthe power back-off determination of the second transceiver. This leadsto asymmetrical links. For example, if the first transceiver requests apower back-off of N dB, and the second transceiver requests a powerback-off of N+6 dB, then the first transceiver transmits 6 dB lesssignal power. Therefore, the second transceiver must operate with asignal to echo/NEXT interference ratio that is 6 dB lower than if bothlink partners were transmitting with the same signal power. The firsttransceiver will have a signal to echo/NEXT ratio that is about 6 dBhigher due to the reduced signal strength of the second transceiver.Therefore, the signal to echo/NEXT ratio imbalance between the twotransceivers is actually 12 dB. This disparity in signal to echo/NEXT orself-interference can cause the link between the first and secondtransceivers to be reliable in one direction, and unreliable in theother direction. Therefore, it is desirable to maintain at least somesymmetry between the signal powers in the two directions of the link.

FIG. 2 shows one example of a master transceiver 210 including a powerback-off controller 212, and a slave transceiver 250 including a powerback-off controller 252, connected by a cable 270 (four twisted pairs ofcopper wire) that can benefit from methods of setting power back-off.

The embodiment of FIG. 2 shows the cable 270 including four twistedpairs of copper. However, it is to be understood that the methods ofsetting power back-off of a master and a slave transceiver can beapplied to many different cable types. For example, the cable 270 can bea copper cable such as CAT5, CAT6, CAT7 or the cable 270 can be a DSL,or a single or multi-mode optical fiber.

As previously described, power back-off can be applied to a transmitter(as located within the transceivers 210, 250) to reduce transmit powerto a level which provides enough signal power for the receiver toproperly operate, but still lower than the maximum available power.Reducing the transmit power level reduces the possibility of interferingwith other transceivers. However, as will be discussed, reducing thetransmit power of transmission signals can make the receiver of thetransmission signals more susceptible to, for example, alien crosstalk.

The determination of which of the transceivers is a master transceiverand which is a slave transceiver can be established during a startupprocess. The master transceiver can be designated as the firsttransceiver, and initiates the power back-off exchange. Ethernettransceivers typically go through an auto-negotiation period thatincludes a series of hand-shakes to establish basic operating criteria.One of the criteria can include determining which transceiver is themaster, and which transceiver is the slave. It is to be understood thatthe master and slave relationship can be determined outside of theauto-negotiation process.

The power back-off (master power back-off) of the master transceiverprovides the slave transceiver with guidance on how to back-off thetransmission power of signals being transmitted from the slavetransceiver to the master transceiver. The master transceivercommunicates the master power back-off to the slave transceiver. Theslave transceiver regulates power of signals transmitted by the slavetransceiver with the master power back-off.

The master power back-off decreases the signal power level of signalstransmitted from the slave transceiver and reduces crosstalk from theslave transceiver to the master transceiver (FEXT), and reduces selfinterference of the slave transceiver (echo and NEXT). A first arrow 212shows echo interference, a second arrow 214 shows NEXT interference anda third arrow 216 shows FEXT interference. Signals from other links canalso couple onto the master and slave transceivers in the form of aliencrosstalk 218 (alien NEXT and alien FEXT).

Increasing the power back-off decreases the transmitted signal power.The power back-off should be limited from being so great that thereceiving transceiver cannot properly receive signals.

The power back-off (slave power back-off) of the slave transceiverprovides the master transceiver with guidance on how to back-off thetransmission power of signals being transmitted from the mastertransceiver to the slave transceiver. That is, the slave transceivercommunicates the slave power back-off to the master transceiver. Themaster transceiver regulates power of signals transmitted by the mastertransceiver with the slave power back-off.

Due to the reasons described above regarding the setting of powerback-off (master and slave) it is desirable that the master powerback-off not be appreciably different than the slave power back-off. Themethods described provide master and slave power back-off settings thatdo not differ radically.

FIG. 3 is a flow chart showing steps of one example of a method ofsetting power back-off of a master transceiver and a slave transceiver.A first step 310 of the method includes the master transceiverdetermining a master power back-off. A second step 320 includes theslave transceiver determining a slave power back-off based on signalsreceived from the master transceiver, and based on the master powerback-off.

Determining the Master Power Back-Off

One example of determining the master power back-off includes the mastertransceiver receiving signals transmitted from the slave transceiver.The master transceiver measures the signal strength of the receivedsignals, and compares the received signal strength with a desiredreceived signal strength. The desired received signal strength can bedetermined from look up table (LUT). The desired signal strength can bedetermined by knowing a desired signal to interference and noise ratio(SINR). Based on the measured and the desired received signal strength,the master transceiver can determine a master power back-off. The mastertransceiver communicates the master power back-off to the slavetransceiver so that the slave can adjust the transmission signal poweraccordingly. Determining the master power back-off includes, to at leastsome extent, determining the master power back-off to ensure a desiresignal quality of signal received by the master transceiver, and tolimit the amount of alien crosstalk with other transceivers. The desiredsignal quality typically includes at least one of signal power, orestimated SNR or BER (bit error rate) or FER (frame error rate) ofreceived signals.

The master power back-off determination can be enhanced by measuring thepower of received signals as a function of frequency. For example, bymeasuring and/or estimating the power spectral density (PSD) of thereceived signals. The PSD can be determined at several key frequencies(for example, at the Nyquist frequency, half the Nyquist frequency and atenth of the Nyquist frequency). A shape of the PSD can provide a basisfor estimating cable lengths which can be used for determining the powerback-off.

Other methods can be used for determining the master power back-off. Forexample, the master transceiver determines the master power back-offbased on a time-domain-reflection procedure. That is, the mastertransceiver can transmit signals and based on of reflections of thetransmitted signals, the master transceiver can determine the masterpower back-off. The determination can be based on lengths of the linksas determined by timing of the reflections. The master transceiver canlook up a power back-off setting based on the estimated length of thelink. Basically, the attenuation is estimated based upon an estimatedlength. Based on the attenuation, a power back-off is selected.

Other embodiments include the master transceiver receiving the masterpower back-off from a user, or a higher level management control layer.Additionally, the master transceiver can base the master power back-ofat least in part based on the slave power back-off determined by theslave transceiver. The master power back-off can be calculated throughsimulation and verified experimentally.

The master transceiver can determine the master power back-off bydetermining a minimum power back-off from signals received from theslave transceiver, and selecting a master power back-off in a range(master power back-off select range) set by the minimum power back-offand a maximum power back-off.

The master transceiver can determine the master power back-off bydetermining a minimum power back-off from signals received from theslave transceiver, and selecting a master power back-off within a rangeof the minimum power back-off and the minimum power back-off plus apredetermined range, wherein the minimum power back-off plus the rangeis not greater than a predetermined maximum allowable power back-off.The range can be pre-determined through simulation and verifiedexperimentally.

Determining the Slave Power Back-Off

Generally, the slave transceiver can use the same methods of determiningthe slave power back-off as the master transceiver, but the slavetransceiver additionally uses the value of the master transceiver powerback-off. For example, one embodiment includes the slave transceiverreceiving the master power back-off, and setting the value of the slavepower back-off to be within a predetermined range of the value of themaster power back-off. This ensures that the slave power back-off andthe master power-off do not deviate from each other too much, therebymaintaining relative symmetry between the power of the transmissionsignals of the master transceiver and the slave transceiver. Determiningthe slave power back-off includes to at least some extent, determiningthe slave power back-off to ensure a desired signal quality of signalreceived by the slave transceiver.

The slave transceiver can determine the slave power back-off bydetermining a minimum power back-off from signals received from themaster transceiver, and selecting a slave power back-off in a range setby the minimum power back-off and a maximum power back-off.

The slave transceiver can determine the slave power back-off bydetermining a minimum power back-off from signals received from themaster transceiver, and selecting a slave power back-off within a rangeof the minimum power back-off and the minimum power back-off plus apredetermined range, wherein the minimum power back-off plus the rangeis not greater than a predetermined maximum allowable power back-off.

For one embodiment, the slave transceiver determining a slave powerback-off based on signals transmitted by the master transceiver, andbased on the master power back-off includes the slave transceiverreceiving the master power back-off, and the slave transceiverdetermining the slave power back-off within a predetermined range of themaster power back-off, based on signals transmitted by the mastertransceiver. If the slave transceiver cannot determine a slave powerback-off value within the predetermined range, then informationregarding this occurrence is stored for future interactions (including,for example, future auto-negotiations) between these link partners.

For another embodiment, the slave transceiver determining a slave powerback-off based on signals transmitted by the master transceiver at aknown power level, and based on the master power back-off includes theslave transceiver determining a first estimate slave power back-offbased on signals transmitted by the master transceiver at a known powerlevel. If the first estimated slave power back-off is within apredetermined range of the master power back-off, then setting the slavepower back-off to be the first estimate slave power back-off. If thefirst estimated slave power back-off is not within the predeterminedrange, then setting the slave power back-off to be within thepredetermined range. If the first estimate slave power back-off is notwithin the predetermined range, then this occurrence can be stored forfuture interactions between master and slave link partners.

FIG. 4 is a plot showing representations of minimum power back-off,maximum power back-off and power back-off ranges.

Maximum Power Back-Off

The maximum power back-off (shown in FIG. 4) provides a maximum level ofattenuation of signals transmitted from either the master or the slavetransceiver. The more a transmission signal is attenuated (backed-off),the less the transmission signal interferes with other transmissionsignals. However, too much attenuation can decrease the signal tonoise/interference ratio to the point that the transmission signalcannot be properly received by a link partner.

Generally, for a given link, a minimum received signal power to ensurereasonable operation of the system can be established. This minimumreceived signal power can be used to limit the maximum power back-off.The maximum power back-off can be determined through simulation, andverified experimentally.

Minimum Power Back-Off

The minimum power back-off (shown in FIG. 4) provides a minimum level ofattenuation of signals transmitted from either the master or the slavetransceiver. FIG. 4 shows a minimum power back-off setting for themaster transceiver and a minimum power back-off setting for the slavetransceiver. The less a transmission signal is attenuated, the morelikely it is that the transmission signal will interfere with othertransmission signals. However, the additional signal power providestransmission signals having a greater SINR, but greater alien crosstalk.

Ranges of Power Back-Off

FIG. 4 shows three possible ranges. A first range is a master powerback-off select range. A second range is the master power back-off rangewhich can be communicated to the slave transceiver. A third range is aslave power back-off select range. The primary purpose for designatingranges is to ensure that the master power back-off and the slave powerback-off do not deviate too much. That is, the selection of rangesprevents the master and slave transceivers from independently selectingpower back-offs that are very different, resulting in the previouslydescribed asymmetric links.

The master power back-off select range provides a range from which themaster transceiver can select a master power back-off. The selectedmaster power back-off can vary with different iterations of master powerback-off selections. As shown in FIG. 4, the master transceiver canselect, for example, a master power back-off “1”.

The master transceiver sends the master power back-off to the slavetransceiver. For example, the master transceiver can send the masterpower back-off “1” of FIG. 4 to the slave transceiver. The slavetransceiver then selects the slave power back-off at least in part basedon the master power back-off.

The slave power back-off selection can be additionally constrained to bewithin a range of the master power back-off. The range can be apredetermined range, or the range can be received from the mastertransceiver in the form of a master power back-off range.

In one embodiment, the master power back-off range is a range that isinitially proposed by the master transceiver along with, for example, aminimum power back-off setting. The slave transceiver can determine, forexample, a minimum power back-off, and calculate a slave power back-offselect range. The slave power back-off select range can be defined by aslave power back-off minimum and a slave power back-off maximum. Theslave transceiver can then select a power back-off within an overlap ofthe master power back-off range and the slave power back-off selectrange as shown in FIG. 4.

If the slave transceiver cannot select a power back-off within the tworanges (the master power back-off range and the slave power back-offselect range), then this information is stored for a future interactionsbetween the transceivers. That is, if the master power back-off rangeand the slave power back-off select range do not overlap, then the slavetransceiver cannot select a power back-off within the two ranges. Thissituation suggests making a reasonable compromise on the ranges. Thecompromise can be made by increasing the power back-off ranges so thatthe slave transceiver is able to re-select the power back-off within themaster power back-off range.

Alternatively, the master transceiver can select a different masterpower back-off within the master power back-off select range (such as,master power back-off “2” as shown in FIG. 4). The different masterpower back-off and associated master power back-off range can providethe slave transceiver with enough overlap between the master powerback-off range and the slave range to make as acceptable slave powerback-off selection.

Interactive Power Back-Off Determinations of the Master and SlaveTransceivers

The determination of which transceiver of link partners is the mastertransceiver and which is the slave transceiver can be determined duringinitialization of the transceivers. For example, for Ethernettransceivers, auto-negotiation establishes a transmission link betweenlink partner transceivers, and can designate a first transceiver as themaster transceiver and designate a second transceiver as the slavetransceiver.

As previously stated, the initialization process (and/or futureinteractions) can benefit from past power back-off interactions betweenthe master and slave transceivers. If a slave power back-off could notbe selected within a range of the master power back-off, the range canbe changed to increase the chances that the slave transceiver is able tore-select within the range. Alternatively, the master transceiver canselect a new master power back-off that is more likely to allow theslave transceiver to select a slave power back-off that is within therange of the master power back-off.

When the slave transceiver is unable to select a slave power back-offwithin the master power back-off range, the slave transceiver can reporta preferred power back-off range to the master. There are severaldifferent tactics the master can use during the next interactionsbetween the master transceiver and the slave transceiver to ensure thatthe master power back-off range and the slave power back-off rangeoverlap.

If the slave power back-off range is above the master power back-offrange (the slave transceiver requires less power back-off than themaster transceiver), the master transceiver can modify the master powerback-off range by either shifting the whole master power back-off rangetoward the slave transceiver power back-off range, or by extending theupper limit of the master power back-off range.

If the slave power back-off is below the master power back-off range(the slave transceiver requires more power back-off than the mastertransceiver), the master transceiver can modify the master powerback-off range by either shifting the whole master power back-off rangetowards the slave power back-off range, or by extending the lower limitof the master power back-off range.

More sophisticated schemes can include determining a gap size betweenthe master power back-off range and the slave power back-off range, andthen shifting both the master power back-off and the slave powerback-off towards each other. This can ensure that both the mastertransceiver and the slave transceiver compromise their desired ranges ina nearly symmetrical way.

Simultaneous Setting of Master and Slave Power Back-Off

One embodiment of master and slave power back-off setting includes themaster transceiver starting regulation of the power of signalstransmitted by the master transceiver simultaneous with the slavetransceiver starting regulation of the power of signals transmitted bythe slave transceiver.

Synchronization between master and slave transceivers is generallyrequired to ensure reliable initialization between the master and slayerlink partners. The link partners should be made aware of each othersstatus. A common method of obtaining synchronization between the masterand slave transceiver link partners is for the transceivers to exchangetransceiver status information and running counter values. The countervalues can be used to allow each transceiver to lock to the runningcounter values. Current status information of a link partner allows fora reliable prediction of next stage and/or action by the link partner.Locking of the running counters of each link partner allows thetransceivers (master and slave) to change states simultaneously and/ortake necessary actions in a timely manner when a link partner changestransmission parameters.

When synchronization has been obtained, each transceiver knows exactlywhen the other transceiver is going to begin regulating transmit signalswith the selected power back-off. Simultaneous application of the masterand slave power back-offs is desirable because situations in which thereis a large transitional imbalance between power back-offs of thetransceivers can be avoided. Simultaneous application of the master andslave power back-offs is also desirable because the transceiverstypically require readjustment of parameters after a transmitter powerchange.

FIG. 5 is a flow chart showing steps of another example of a method ofselecting master and slave transceiver back-offs. A first step 510includes initializing (including handshaking exchanges between thetransceivers) and designating link partners as master and slavetransceivers. A second step 520 includes the master transceiverselecting the master power back-off. The power back-off ranges, minimumand/or maximum can be specified at this point. If power back-offs havebeen determined previously, knowledge of the previous determinations canbe used to modify or update the specified ranges, minimum and/or maximumpower back-offs. A third step 530 includes the slave transceiverselecting the slave power back-off within the specified ranges. A fourthstep 540 includes determining whether the slave transceiver was able toselect a slave power back-off within specified ranges (for example,within the overlap shown on FIG. 4). If not, the initialization (or anext interaction which does not require a full initialization) isexecuted again with different ranges or a different master powerback-off selection. If the slave is able, to select a slave powerback-off within the specified ranges, a fifth step 550 includes themaster and slave transceivers setting the power back-offs.

FIG. 6 is a flow chart showing steps of one example of a method ofsetting power back-off of a slave transceiver. A first step 610 includesthe slave transceiver receiving a master power back-off from a mastertransceiver. A second step 620 includes the slave transceiverdetermining a slave power back-off based on signals transmitted by themaster transceiver at a known power level, and based on the master powerback-off.

One example of slave transceiver determining a slave power back-offbased on signals transmitted by the master transceiver, and based on themaster power back-off includes the slave transceiver determining theslave power back-off within a predetermined range of the master powerback-off, based on signals transmitted by the master transceiver. If theslave transceiver cannot determine a slave power back-off value withinthe predetermined range, then information regarding this occurrence isstored for future interactions between master and slave link partners.If the slave transceiver cannot determine a slave power back-off valuewithin the range, then information regarding this occurrence is storedfor future interactions (which could be, for example, auto-negotiations)between master and slave link partners.

One other example of the slave transceiver determining a slave powerback-off based on signals transmitted by the master transceiver, andbased on the master power back-off includes the slave transceiverdetermining a first estimate slave power back-off based on signalstransmitted by the master transceiver. If the first estimated slavepower back-off is within a predetermined range of the master powerback-off, then setting the slave power back-off to be the first estimateslave power back-off. If the first estimated slave power back-off is notwithin the predetermined range, then setting the slave power back-off tobe within the predetermined range.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the appended claims.

What is claimed:
 1. A method of setting power back-off of a mastertransceiver and a slave transceiver, the master transceiver and theslave transceiver being connected by a cable to define a channel, themethod comprising: the master transceiver determining a master powerback-off; the slave transceiver determining a slave power back-off basedon signals received from the master transceiver, and within apredetermined range of the master power back-off such that substantialrelative signal power symmetry is established between signalstransmitted between the master and the slave transceivers along thechannel.
 2. The method of claim 1, further comprising executing anauto-negotiation establishing a transmission link between link partnertransceivers in which a first transceiver is designated as the mastertransceiver and a second transceiver is designated as the slavetransceiver.
 3. The method of claim 1, wherein the master transceiverregulates power of signals transmitted by the master transceiver withthe slave power back-off, and wherein the slave transceiver regulatespower of signals transmitted by the slave transceiver with the masterpower back-off.
 4. The method of claim 3, wherein the master transceiverstarts regulation of the power of signals transmitted by the mastertransceiver simultaneous with the slave transceiver starting regulationof the power of signals transmitted by the slave transceiver.
 5. Themethod of claim 1, wherein determining the master power back-offcomprises the master transceiver determining a minimum power back-offfrom signals received from the slave transceiver, and selecting a masterpower back-off in a range set by the minimum power back-off and amaximum power back-off.
 6. The method of claim 1, wherein determiningthe master power back-off comprises the master transceiver determining aminimum power back-off from signals received from the slave transceiver,and selecting a master power back-off within a range of the minimumpower back-off and the minimum power back-off plus a predeterminedrange, wherein the minimum power back-off plus the range is not greaterthan a predetermined maximum allowable power back-off.
 7. The method ofclaim 1, wherein determining the slave power back-off comprises theslave transceiver determining a minimum power back-off from signalsreceived from the master transceiver, and selecting a slave powerback-off in a range set by the minimum power back-off and a maximumpower back-off.
 8. The method of claim 1, wherein determining the slavepower back-off comprises the slave transceiver determining a minimumpower back-off from signals received from the master transceiver, andselecting a slave power back-off within a range of the minimum powerback-off and the minimum power back-off plus a predetermined range,wherein the minimum power back-off plus the range is not greater than apredetermined maximum allowable power back-off.
 9. The method of claim1, wherein the master transceiver determines the master power back-offbased on signals received from the slave transceiver.
 10. The method ofclaim 1, wherein the master transceiver determines the master powerback-off based on cable length estimations.
 11. The method of claim 1,wherein the master transceiver determines the master power back-offbased on a time-domain-reflection procedure.
 12. The method of claim 1,wherein the master transceiver determines the master power back-off by auser supplying a master power back-off value.
 13. The method of claim 1,wherein the master transceiver determines the master power back-off by ahigher level Ethernet management control layer supplying a master powerback-off value.
 14. The method of claim 1, wherein the slave transceiverdetermines the slave power back-off additionally based upon a powerback-off range received from the master transceiver.
 15. The method ofclaim 1, wherein the slave transceiver determining a slave powerback-off based on signals received from the master transceiver, andbased on the master power back-off comprises: the slave transceiverreceiving the master power back-off; the slave transceiver determiningthe slave power back-off within a predetermined range of the masterpower back-off, and based on signals transmitted by the mastertransceiver.
 16. The method of claim 15, wherein if the slavetransceiver cannot determine a slave power back-off value within thepredetermined range, then information regarding this occurrence isstored for future interactions between these link partners.
 17. Themethod of claim 15, wherein if the slave transceiver cannot determine aslave power back-off value within the range, then information regardingthis occurrence is stored for future auto-negotiations between theselink partners.
 18. The method of claim 1, wherein the determining themaster power back-off comprises determining the master power back-off toensure a desire signal quality of signal received by the mastertransceiver.
 19. The method of claim 1, wherein the determining theslave power back-off comprises determining the slave power back-off toensure a desire signal quality of signal received by the slavetransceiver.
 20. The method of claim 1, wherein the slave transceiverdetermines the slave power back-off based on signals transmitted fromthe master transceiver at a known power level.
 21. The method of claim 1wherein the power back-off for the slave is set within 6 dB of the powerback-off for the slave to obtain the substantial relative signal powersymmetry.
 22. A system comprising: a master transceiver; a slavetransceiver, the slave transceiver connected to the master transceiverthrough a cable; the master transceiver comprising means for determininga master power back-off; the slave transceiver comprising means fordetermining a slave power back-off based on signals received from themaster transceiver, and based on the master power back-off such thatsubstantial relative signal power symmetry is established betweensignals transmitted between the master and the slave transceivers alongthe channel.